1. Field of Use
The present invention relates to a digital apparatus for the recovery of binary information recorded on magnetic media, such as disks, diskettes and tapes.
2. Prior Art
Among the methods of recording binary information on magnetic media, the most used is frequency modulation (FM) also known as single density and modified frequency modulation (MFM) or double density. Successively read out binary 1 and 0 bits recorded in FM are identified by the presence or absence, respectively, of a pulse in the center of contiguous recorded cells.
Each cell is defined by two timing pulses. The first one defines the beginning of the cell and the second one defines the beginning of the subsequent cell.
The time interval of a cell varies according to the media used. For instance, in the case of an 8 inch diskette and FM recording, the cell has a nominal length of 4 microseconds. Therefore, the nominal time intervals between two subsequent pulses may be 2 microseconds or 4 microseconds.
In case of MFM according successively read out binary 1 or 0 bits are also identified by the presence or absence respectively of a pulse in the center of contiguous cells.
However, MFM recording differs from FM recording in that the timing pulse, defining the beginning of a cell, is absent when a pulse representative of a binary 1 information is present in the center of such cell or in the preceding one.
For MFM also, the time interval of the cell depends on the recording media. For instance, in the case of an 8 inch diskette and MFM recording, the nominal length of the cell is 2 microseconds. Therefore, the nominal interval time between two subsequent pulses can be 2, 3 or 4 microseconds. Further, information on FM and MFM recording methods can be found in the IBM document GA 21-9257-1 entitled "IBM Two Side Diskette Original Equipment Manufacturers Information--Second Edition", dated November, 1977.
The pulse sequence read out from the magnetic medium support is applied to an input of a recovery system which supplies to an output, the binary information related to the input pulse sequence. Such pulse sequence periodically includes a so-called synchronization field (generally of 6 or 12 bytes), containing a predetermined number of pulses corresponding to a plurality of contiguous cells in which all "1" information bits or all "0" information bits have been recorded.
The synchronization field is used by the recovery system for locking in and for establishing if a pulse detected in the input is a timing pulse or a pulse representative of a recorded information bit. The recovery system is therefore able to correctly detect information recorded on the magnetic medium on the basis of the time interval between two subsequent pulses and the nature of such pulses.
Unfortunately, data recovery through only measurement of the time interval between subsequent pulses is not reliable, since such interval may present a substantial deviation from its nominal value resulting in the misinterpretation of the pulse sequence during the recovery phase.
Such deviations result from two main causes. The first is due to speed changes in the magnetic media, that is, in the rotational speed tolerances of the motor which drives the magnetic media. The second is due to the so-called phenomenon of peak-shift of the recorded pulse. As is well known in the art, such shift is primarily due to the mutual influence of adjacent pulses.
As shown, such shift can be considered zero only when the recorded pulse density is constant, that is, if the interval between subsequent pulses is always equal.
Clearly, such situation does not exist in FM and MFM recording except for the synchronization field. Therefore, the recovery system needs apparatus to correct for the causes of such errors. The most known of these apparatus are those which make use of phase lock circuits. These circuits receive the pulse sequence as an input and they supply, as an output, a so-called square wave window signal which is continuously synchronized with the input pulses. In other words, the window signal is modified in order to maintain a preestablised phase relationship with the input pulses.
The window signal and the pulse sequence are then applied to a logical network of the recovery system and suitably decoded. The read out pulse occurring when the window signal is at a first electrical level is interpreted as an information bit. A read out pulse occurring when the window signal is at a second electrical level is interpreted as a timing pulse.
The phase lock circuits generally comprise a phase comparator which receives as an input, the pulse sequence and the window signal and supplies as an output, a signal proportional to their difference in phase. Such signal is applied, through suitable filters, to a voltage controlled oscillator (VCO) which generates the window signal. These circuits, besides frequent and expensive trimming, are often affected by stability problems.
Digital phase lock apparatuses have been recently suggested to overcome such diadvantages. For instance, U.S. Pat. No. 4,357,707 discloses digital phase lock apparatus to be used together with a diskette digital controller for recovering information recorded both in FM and in MFM. According to such patent, the nominal interval between two window signal transitions is given by the sum of a variable number of periods of a fixed frequency timing signal. The circuit counts the number of timing signal periods occurring between the last window signal transition and the instant at which the next following read out pulse is received. Based on such information, it changes the position of the subsequent window signal transition, in order that the read out pulse is centered between the two window signal transitions.
In case of MFM recording, the patented apparatus varies the length of the interval between two window signal transitions upon the reception of a nth pulse, not only taking into account the interval between the last transition of the window signal and such nth pulse, but also the interval between the (n-1).sup.th pulse and the window signal transition immediately preceding such (n-1).sup.th pulse.
In this way, the patented apparatus tries to recover from the error caused by the "peak shift" phenomenon which especially affects the MFM recording. Additionally, the apparatus in U.S. Pat. No. 4,357,707 attempts to recover from the error caused by the magnetic media speed variation. It continuously verifies whether each pulse of a plurality of pulses received as an input is early or late relative to its nominal position. Accordingly, it increments or decrements a counter and modifies the number of timing periods defining the nominal interval between two window signal transitions as a function of the status of such counter.
Besides requiring the use of high performance components, intrinsically expensive (such as control memory with minimum access time), the patented apparatus is affected by several limitations in the recovery of MFM recorded binary information. In fact, even though the recovery of the errors due to speed changes and to the "peak shift" phenomenon is carried out by independent circuits, such circuits are responsive to a pulse train in which these errors are both present and add to each other. It is therefore clear that recovery from the error due to the speed variation is affected by the "peak shift" phenomenon as well as the recovery from the error due to the "peak shift" phenomenon is affected by the magnetic media speed error.
Therefore, a correct phase relation between window signal and the read out pulse train can not be assured. Additionally, presuming a correct recovery of the error due to speed variation, the circuit disclosed in U.S. Pat. No. 4,357,707 is able to recover from errors due to the peak shift phenomenon whose maximum value is often less than that which may actually occur. As concerns the peak shift recovery, it must be added that the patented circuit tends to propagate the indeterminate error or measurement uncertainty which affects the measurement of the interval between a pulse and the previous window signal transition onto the subsequent intervals. The digital apparatus of the present invention overcomes such disadvantages.